Through-thickness inhomogeneity in microstructure and tensile properties and tribological performance of friction stir processed AA1050-Al₂O₃ nanocomposite

Abstract

Friction stir processing (FSP) is used to fabricate Al-Al₂O₃ nanocomposite by addition of Al₂O₃ nanoparticles into a groove in AA1050 aluminum alloy and being processed using FSP. Evolution of grain structure, distribution of the Al₂O₃ nanoparticles, tensile properties and tribology are investigated in four samples extracted from each 2 mm of the fabricated composite, respectively from top surface. It was found that the finest grain structure formed in the second region from the top surface due to the most efficient stirring in this region. In addition, the presence of nanoparticles with an efficient distribution in this region may inhibit grain growth and lead to an eventually fine grain structure. After one pass of FSP, nanoparticles were found to be efficiently distributed in the top surface and less uniformly distributed in other regions leading to formation of particle depleted regions (PDRs) and coarse agglomerated particles. Fracture in the tensile test samples changed from ductile to brittle in the sample with agglomerated particles. Application of the second cycle of FSP leaded to further grain refinement, more uniform distribution of nanoparticles and less agglomerated coarse particles and consequently leaded to more uniformity in tensile properties and tribology behavior of the samples. This was the most effective on grain refinement for the regions which were not efficiently refined or with more extended PDRs in the first cycle. Coefficient of friction (CoF) of aluminum was significantly reduced with addition of Al₂O₃ particles which was attributed to increase in strength and reduction in sliding contact area.

Introduction

Due to the high existing demands for reducing energy costs, industrial applications of metals and alloys with high specific strength is growing fast. Aluminum and its alloys have been one of the most suitable cases for this purpose as they provide reasonably high specific strength but relatively low costs and easiness of production. However, these alloys suffer from poor tribological properties, e.g., surface hardness and wear resistance, which limit them in application. Therefore, aluminum has been composited with many different hardening particles, e.g., SiC [1], SiCp [2], Al₂O₃ [3], CNT [4], using different approaches, e.g., Friction stir processing (FSP) [5], accumulative roll bonding [6], vortex method [7], infiltration technique [8] and casting [9]. It should be noted that it is not always necessary to produce a bulk composite as only surface hardening can suffice to pass requirements. In addition, aluminum alloys are very likely to go through a final metal forming process prior to application while microstructure and heat treatment [10] and hard particles or additives can significantly degrade the workability of the produced nanocomposites [11]. Therefore, surface composites of aluminum may find unique applications as they provide possibility to benefit from good surface hardness and wear resistance while withholding the main matrix properties and yet acquire reasonable workability.

Ceramics micro and nanoparticles are the most likely used ones for the purpose of compositing. This is because such composites provide acceptable ductility and improved strength, wear and creep resistance [12]. It has been found that using nanosized ceramics particles can improve wear resistance and result in a more uniform distribution of the hardening particles [13]. For example, addition of carbon nanotubes can result in an improvement in electrical and heat conductivity in addition to optical and dielectric properties [14].

There are different methods for producing metal matrix composites, e.g., Friction stir processing (FSP) [5], accumulative roll bonding [6], vortex method [7], infiltration technique [8] and casting [9]. In casting methods, avoiding agglomeration, particle growth and formation of detrimental phases are difficult, the latter may be due to reaction between the particles and the melt [15]. Indeed, all of these restrict formation of a homogeneous composite while may be avoided using solid state processes [16,17]. There are different solid state methods which are mostly used for producing surface composites, e.g., FSP [5], and ARB [6], FSP has also been used for production of MMCs [5], especially for light metals, e.g., Al/SiC [18,19], CNT [[20], [21], [22]], TiC [23,24] and Al₂O₃ [[25], [26], [27]]. There are many processing parameters such as the pin geometry, linear and rotational speed of the pin and the number of FSP cycles that play role in the FSP process [28,29]. Few research activities have been focused on the effect of number of passes on the distribution and size of the particles produced by FSP and how they affect mechanical properties of the MMCs [4,5,9,[21], [22], [23],25]. Agglomeration of the hardening particles is a big issue in production of these composites which degrades them in mechanical properties and performance. It has been observed by Sharifitaba et al. [30] that the size of the agglomerated nanoparticles reduces from 650 to 70 nm with increasing the number of FSP cycles from 1 to 4. In addition, as increasing the amount of deformation can result in more significant recrystallization and grain refinement, the grain size reduces with increasing the number of FSP passes [23,31]. In fact, increasing the number of passes results in more homogeneous distribution of nanoparticles in the matrix but as well reduction of the grain size, both of which, yield improvements in mechanical properties [30,32]. It should be noted that the nanoparticles have a determining role in inhibition of grain growth and help with achievement of a finer grain structure [25].

Despite of the existing research results in this field, it is essentially necessary to systematically investigate the distribution of nanoparticles through thickness of a nanocomposite produced by FSP. In addition, it is interesting to understand how mechanical properties vary through thickness with variations in microstructure. Indeed, it is necessary to make solid and interrelated conclusions on the evolution of microstructure as a result of FSP, but as well to know how it is affected by distribution of nanoparticles which can inhibit grain growth. In addition, one may need to understand how these factors affect tensile and tribology properties. These questions and few other sub-questions constitute the structure of the research presented hereafter.